Update defaults to gpt-5.1 (#6652)

## Summary
- update documentation, example configs, and automation defaults to
reference gpt-5.1 / gpt-5.1-codex
- bump the CLI and core configuration defaults, model presets, and error
messaging to the new models while keeping the model-family/tool coverage
for legacy slugs
- refresh tests, fixtures, and TUI snapshots so they expect the upgraded
defaults

## Testing
- `cargo test -p codex-core
config::tests::test_precedence_fixture_with_gpt5_profile`


------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_6916c5b3c2b08321ace04ee38604fc6b)

codex-rs/core/tests/suite/tool_parallelism.rs

@@ -57,9 +57,9 @@ async fn run_turn_and_measure(test: &TestCodex, prompt: &str) -> anyhow::Result<
 #[allow(clippy::expect_used)]
 async fn build_codex_with_test_tool(server: &wiremock::MockServer) -> anyhow::Result<TestCodex> {
     let mut builder = test_codex().with_config(|config| {
-        config.model = "test-gpt-5-codex".to_string();
+        config.model = "test-gpt-5.1-codex".to_string();
         config.model_family =
-            find_family_for_model("test-gpt-5-codex").expect("test-gpt-5-codex model family");
+            find_family_for_model("test-gpt-5.1-codex").expect("test-gpt-5.1-codex model family");
     });
     builder.build(server).await
 }